Apoptosis (programmed cell death) plays a central role in the development and homeostasis of all multi-cellular organisms. Alterations in apoptotic pathways have been implicated in many types of human pathologies, including developmental disorders, cancer, autoimmune diseases, as well as neuro-degenerative disorders, and retinal degradation. It is a tightly regulated pathway governing the death processes of individual cells and can be initiated either extrinsically or intrinsically. The latter is an intracellular mechanism triggered by the mitochondria while the former involves the interaction of a ‘death receptor’ with its corresponding ligand at the cell membrane.
Thus, the programmed cell death pathways have become attractive targets for development of therapeutic agents. In particular, since it is conceptually easier to kill cells than to sustain cells, attention has been focused on anti-cancer therapies using pro-apoptotic agents such as conventional radiation and chemotherapy. These treatments are generally believed to trigger activation of the mitochondria-mediated apoptotic pathways. However, these therapies lack molecular specificity, and more specific molecular targets are needed.
Retinal detachment (RD), defined as the separation of the neurosensory retina from subjacent retinal pigment epithelium (RPE), results in the apoptotic death of photoreceptor cells (Cook et al. 1995; 36(6):990-996; Hisatomi et al. Curr Eye Res. 2002; 24(3):161-172; Zacks et al. Invest Ophthalmol Vis Sci. 2003; 44(3):1262-1267. Yang et al. Invest Ophthalmol Vis Sci. 2004; 45(2):648-654; herein incorporated by reference in their entireties). Rodent and feline models of RD have demonstrated the activation of pro-apoptotic pathways nearly immediately after the retina becomes separated from the RPE (Cook et al. 1995; 36(6):990-996; Hisatomi et al. Curr Eye Res. 2002; 24(3):161-172; Zacks et al. Invest Ophthalmol Vis Sci. 2003; 44(3):1262-1267. Yang et al. Invest Ophthalmol Vis Sci. 2004; 45(2):648-654; herein incorporated by reference in their entireties). Histological markers of apoptosis such as terminal deoxynucliotidyl transferase nick end label (TUNEL) staining reach a peak at approximately three days after RD, with apoptotic activity and progressive cell death persisting for the duration of the detachment period. Clinical experience in the repair of retinal detachments, however, has demonstrated that there is a window of opportunity for repair with preservation of good visual acuity. Retrospective case series have demonstrated that significant numbers of patients with macula-off RDs repaired within 5-10 days after onset of detachment can retain relatively good visual function, but that the visual acuity drops significantly as the time between detachment and repair extends (Burton. Trans Am Ophthalmol Soc. 1982; 80:475-497; Ross et al. Ophthalmology. 1998; 105(11):2149-2153; Hassan et al. Ophthalmology. 2002; 109(1):146-152; herein incorporated by reference in their entireties). The delayed time between the activation of pro-apoptosis pathways and the clinical onset of visual loss suggests that intrinsic neuroprotective factors may become activated within the neural retina, and may serve to counter-balance the effects of the pro-apoptotic pathways activated by retinal-RPE separation.